PART- I
In the body of any organism there are two
types of characters i.e. qualitative and quantitative .The characters which are
governed by one or few genes are called oligogenic
or qualitative character and those which are
governed by numerous genes are called polygenic
or quantitative character.
QUANTITATIVE
AND QUALITATIVE CHARACTERS IN PLANT BREEDING
Quantitative characters are also known as
metric characters. They are controlled by many
genes. Such characters are agronomic in nature and have complex inheritance.
They are easily influences by the environment. They are continuous in their
phenotypic expression and they segregate in the second filial generation F2.
Such characters cannot be grouped into distinct classes but they can be
improved by recurrent selection leading to small but steady genetic gain in
each selection cycle. They are responsive to high temperature and water and are
physiological in nature. Examples are grain yield, total dry matter, plant
height, protein and oil content in plants and general disease resistance.
Qualitative characters are traits that show simple Mendelian
inheritance. They are controlled by one or two pairs of genes. They are easily
measured and their phenotype in the second filial generation F2 has close
resemblance with their parents. E.g. flower or leaf colour, leaf shape, fruit
shape, etc. It should be noted that selection is faster when dealing with
qualitative traits compared with quantitative traits because unfavourable
traits can be easily eliminated.
P= G+
E
V2p =
V2g + V2e
Where
P =Phenotype, G = genotype and E = environment, V2 = variance
HERITABILITY
Heritability
is a measure of the genetic contribution to phenotypic variability
Types of heritability:
Broad-Sense
Heritability: this expresses that proportion of
variance due to the genetic component:
H2 =
VG/VP
Where
VG is the genetic variance and VP is the phenotypic variance
Narrow-Sense
Heritability:
h2
=VA/VP
http://www.unaab.edu.ng
Because
VP =VE + VG and VG = VA + VD, we obtain:
h2 =
VA/ VE + VA + VD
Where
VA =additive variance, VD =dominance variance,
Example
The mean and variance of corolla length
in two highly inbred strains of Nicotiana and their progenies are shown in
table below. One Parent (P1) has short corolla length and the other (P2) has a
long length.
Strain
Mean (mm) Variance
P1
40.47 3.12
P2
93.75 3.87
F1
(P1xP1) 63.90 4.47
F2
(F1xF1) 68.72 47.70
Calculate
the heritability for corolla length
Solution
H2 =
VG/VP, V2p = V2g + V2e
Because the strains breed true, they are
assumed to be homogenous and the variance 3.12 and 3.87 is considered to be as
a result of the environmental influences. The average = (3.12 + 3.87)/2 = 3.50 F1
is also genetically homogenous; hence it gives us an additional estimate of the
environmental factors. By averaging over the two parents, we have:
(3.50
+ 4.47)/2 =4.12
V2p =
V2g + V2e
47.70
= V2g + 4.12
V2g =43.58
H2 =
VG/VP,
=43.58/47.70
=0.91
=91%
This
implies that about 91% of the variation in corolla length is due to genetic
influences.
By== Mukesh Kumar
By== Mukesh Kumar
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